The present invention relates to an apparatus for performing operations such as welding, cutting, evaporating, engraving or machining or other surface modification of any material with an electron beam. More particularly, the invention relates to an electron beam welder for welding a workpiece in a non-vacuum environment.
Electron guns for generating focussed, high energy electron beams are well known. In order to prevent electrons from colliding with gas molecules, which causes the beam to disperse, the beam is generated in a vacuum. This is also necessary to prevent arcing in the gun region and to increase the life of the cathode. Likewise, the workpiece to be welded or otherwise operated on by the electron beam is preferably located in an evacuated environment. Since beam dispersion decreases with lower pressure, the distance between the gun assembly and the workpiece may be increased. However, the welding of workpieces in a reduced pressure environment requires evacuating the workpiece chamber each time a new workpiece or batch of workpieces is introduced or necessitates that some form of pressure lock scheme be employed for providing continuous part throughput.
Electron beam welding in a non-vacuum environment offers the advantage of eliminating the workpiece chamber and the necessity of repeated evacuation; the size of the workpiece is not limited by the size of a chamber, and continuous welding is possible. However, the distance between the electron beam generator and the workpiece must be kept to a minimum, and a sequential series of vacuum stages separated by orifices is necessary.
U.S. Pat. Nos. 4,304,979 and 4,393,295 disclose electron beam generators for operating on workpieces in a non-vacuum environment. Each generator has three vacuum stages, including a first stage next to a lower orifice from which the beam exits, a second or intermediate stage connected to the first stage via a middle orifice, and a high vacuum stage connected to the second stage via an upper orifice. The electron beam is generated in the high vacuum stage, and then passes through the axially aligned orifices, which are as small as possible in order to limit influx of air along the beam path.
U.S. Pat. Nos. 3,171,943 and 3,444,350 disclose electron beam generators for operating on workpieces in a non-vacuum environment, and address the problem of beam attenuation by pumping a high pressure stream of gas across the exit orifice to deflect any incoming air.
FIG. 1 depicts a prior art electron beam welding apparatus for welding a workpiece in a non-vacuum environment. This particular apparatus is sold by PTR-Precision Technologies of Enfield, Conn. The exterior of the apparatus pictured is formed by lead shrouding 2 with an access door 4 for access to the electron gun assembly 12 positioned in high vacuum housing 6. The housing 6 is formed by an upper section 8, a middle section 9, and a lower section 10. A high voltage cable 13 is connected to the gun through an insulated cable socket 14 extending into a cavity formed in the upper and middle housing sections.
An electron beam generated by the gun assembly is directed downward along an axial path through a focussing column 16 which is surrounded by a beam alignment coil 18 and a focussing coil 20 situated in the lower section 10 of the high vacuum housing. The column 16 is formed by an upward extension of an orifice housing 22 which houses the orifices 24, 25, 26 separating sequential vacuum stages from the outside environment. The highest vacuum ends at upper orifice 24 at the lower end of the focussing column 16. This is followed by a second or intermediate stage vacuum duct 30, whence the beam passes through middle orifice 25 into first stage vacuum duct 28, and finally exits through lower orifice or nozzle 26 to the ambient environment.
The first stage duct 28 is evacuated by a first stage pumping line 29, while the second stage duct 30 is evacuated by a second stage pumping line 31. The degree of vacuum increases, i.e., the pressure drops, between the first stage duct 28 and the cavity in the high vacuum housing 6, which is evacuated by a separate line (not shown). Since the electron beam emerges from an orifice 26 between a vacuum and a non-vacuum environment, air molecules tend to rush in along the path of the beam, causing dispersion. This influx of air is diverted somewhat by a jet of air which is propelled laterally from the external airjet 32 across the lower orifice.
All of the cited prior art electron beam welders have flat surfaces in the vicinity of the beam exit orifice, which makes them suitable only for readily accessible weld joints. For joints which are difficult to access, the distance between the exit orifice and the workpiece would be too large, and scattering of the electrons would result due to the presence of too many intervening air molecules.